During operation of discharge lamps, which will also be referred to below as lamp for short, there is the phenomenon of the growth of electrode peaks. Material which is vaporized by the electrodes at one point is again deposited at preferred points on the electrode and this results in the formation of electrode peaks. These electrode peaks initially have the advantage that the plasma arc of the arc discharge produced in the lamp finds a stable attachment point on the electrode and does not jump between a plurality of attachment points. This jumping of the discharge point is also referred to as arc jumping and is expressed in flicker of the lamp. This is particularly destructive if the light from the lamp is used for projecting images.
The attachment point of the arc is only formed on an electrode which acts as a cathode. The attachment of the arc on the anode is flat. In the case of lamps which are operated on alternating current, the arc jumping is therefore a widespread problem since, with each change in polarity, the arc needs to find an attachment point on the electrode changing from anode to cathode. The above-described electrode peaks form a preferred attachment point for the arc and thus reduce arc jumping.
However, problems can also arise as a result of the electrode peaks. Under unfavorable conditions, two or more electrode peaks may form. It may then arise that the arc attachment jumps between the different electrode peaks.
In the document EP 1 624 733 A2 (Suzuki), this problem is solved by virtue of the fact that the operating frequency, i.e. the frequency of the alternating current at which the lamp is operated, is reduced for a limited time. This method functions by virtue of the fact that an electrode is heated while it is acting as the anode and is cooled while it is acting as the cathode. This results in a temperature fluctuation with a time profile which corresponds to the operating frequency. At high frequencies, a mean temperature is set as a result of the thermal capacity of the electrodes. During so-called square-wave operation, the lamps are operated on a square-wave current with a frequency of typically from 200 to 5000 Hz. Depending on the design of the lamp, noticeable temperature modulation of the electrodes can arise even at 200 Hz. In the case of considerable temperature modulation, the electrode reaches temperatures at which the superfluous electrode peaks are fused off.
A further problem arises if the electrode peaks become too long and therefore the plasma arc between the electrodes becomes short. The shortening of the plasma arc results in a reduced running voltage of the lamp. Since the lamp is generally regulated to a constant power, the reduced running voltage results in an increased current. This is disadvantageous for the loading of component parts in the control gear for the lamp. Component parts which are involved in the provision of the lamp current need to be designed for an increased lamp current or will become subject to damage.
Video projectors often require a light source which has a temporal sequence of different colors. As is described in the document U.S. Pat. No. 5,917,558 (Stanton), this can be achieved by a rotating color wheel, which filters changing colors from the light from the lamp. The time periods during which the light assumes a specific color do not necessarily need to be the same. Instead, a desired color temperature which results for the projected light can be set via the ratio of these time periods in relation to one another.
Generally, the lamp is operated on a square-wave lamp current. The reciprocal value of the period duration of the square-wave lamp current is understood to be the abovementioned operating frequency. In the prior art, the lamp current is generated from a DC source with the aid of a commutation device. The commutation device conventionally comprises electronic switches, which commutate the polarity of the DC source in step with the square-wave lamp current. During the commutation, overshoots in practice cannot be avoided completely. Therefore, in the prior art, the time at which a commutation is intended to take place is combined with the time at which the color of the light changes in order to suppress the overshoots. For this purpose, a sync signal is provided which has a sync pulse in synchronism with the abovementioned color wheel. The color change and the commutation of the lamp current is synchronized with the aid of the sync signal.
In the prior art according to EP 1 624 733 A2 (Suzuki), the operating frequency is lowered, as explained above, for a limited time in order that additional electrode peaks are fused off. This results in the problem that the synchronization of the commutation with the sync signal is no longer ensured by the reduction in frequency. The commutation can therefore also take place at a time at which there is no color change taking place.